Ultrahigh-frequency apparatus



Aug. 15, 1961 J. H. CHRISTENSEN ETAL 2,996,692

ULTRAHIGH-FREQUENCY APPARATUS 2 Sheets-Sheet 1 Filed May 2, 1960 INVENTORS.

J. H. CHRISTENSEN EI'AL 2,996,692

ULTRAHIGH-FREQUENCY APPARATUS Aug. 15, 1961 2 Sheets-Sheet 2 Filed May 2, 1960 $6 I INVENTORS United States Patent 2,996,692 ULTRAHIGH-FREQUENCY AIPARATUS James H. Christensen, Athens, and Anthony J. Sm th, Florence, Ala., assignors to Tennessee Valley Authority, a corporation of the United States Filed May 2, 1960, Ser. No. 26,385 2 Claims. (Cl. 333-98) (Granted under Title 3'5, U.S. Code (1952), see. 266) The invention herein described may be manufactured and used byor for the Government for governmental purposes without the payment to us of any royalty therefor.

Our invention relates to ultrahigh-frequency electrical apparatus, and more particularly to improvements in standing wave detectors using traveling probes.

Heretofore, in the construction of conventional standing Wave detectors, a narrow slot has been provided in the Wall of the section of wave guide or in the outer conductor of the section of coaxial line (both of which will hereinafter be termed transmission line). This narrow slot permits a pickup device, such as a probe or loop,

to be projected into the section of transmission line so as that the efiects of a longitudinal slot in the conductor wall of a transmission line is to change the characteristic impedance of the line but slightly, and to introduce but a negligible loss by radiation. This effect is ordinarily assumed to be immeasurably small on the basis that when the wall thickness is compared with that of the slot width the slot acts as a wave guide far beyond cutoff for the penetrating field. Thus, in the instance where the penetrating field is thought of as a TM-wave in the parallelplate transmission line formed by the slot sides, the assumption has been that the attenuation of this wave in a direction normal to the axis of the wave guide is so rapid that the coupling to the exterior region is of no practical importance.

Prior art arrangements of the above-mentioned type have proved to be operative; however, we have found that to ensure the highest practical degree of reproducibility and accuracy in determining standing wave ratios of voltage or current within a length of transmission line, the slotted section assembly or slotted line itself cannot be treated as a uniform-loss transmission line. Although it has been assumed in the prior art that the radiation through a narrow slot, properly proportioned and properly positioned, is so negligible as to be imrnaterial in practical use, we have found that this assumption in many instances is not tenable. The radiation through the slot, however well designed, is measurable, and the elimination of such radiation will increase the reproducibility and accuracy of slotted-section measurements.

Thus, in the instance wherein slotted-section assemblies .are used in such applications as the determination of dielectric-constants and loss factors, we have found that radiation from the slot itself causes the slotted section to exhibit nonlinear characteristics which increase the complexity and errors of precision measurements,

Another source of error and a contributing factor to erratic operation of conventional standing wave detectors .has been found to result from reflections of the electric .field off nearby metallic objects and the subsequent re'-' 2,996,692 Patented Aug. 15, 1961 radiation of the ultrahigh frequency electromagnetic field outside the transmission line back into the wave guide via the slot. Such re-radiation may easily change the electric field strength within the transmission line, either adding to or subtracting from it, depending upon the phase of the reflected field. This re-radiation is extremely troublesome, since the amount of metallic reflection surface outside of the wave guide and its distance from the slot vary according to the type of auxiliary equipment necessary during any particular series of measurements. Thus, successive field strength readings under otherwise similar conditions may vary considerably because of variations in the amount of re-radiation through the slot.

Our invention is directed to a standing wave detector (hereinafter termed slotted-section assembly) of improved construction over that of conventional detectors.

We have overcome the difficulties inherent in apparatus of this type in the prior art to a substantial extent in the present invention by providing a standing wave detector having a moving pickup device of the slottedsection type in which substantially all of the slot is covered or closed in with a material which prevents radiation of energy through the slot. Furthermore, several new, advantageous features over conventional standing wave detectors are realized by the present invention.

Among these advantageous features are increased average probe excitation and, in the instance of a wave guide terminated with a shorting plate, a direct proportionality between the magnitude of the inverse standing wave ratio and the length of the wave guide.

It is therefore an object of the present invention to provide a standing wave detector having a movable pickup device of the slotted-section type in which radiation from the slot is substantially eliminated.

Another object of the present invention is to provide a standing wave detector having a movable pickup device of the slotted-section type which exhibits substantially linear loss characteristics.

Still another object of the present invention is to provide a standing wave detector having a movable pickup device of the slotted-section type which is capable of increased reproducibility and accuracy in measuring standing Wave ratios of voltage or current and points of maximum and minimum voltage within a length of transmission line.

In carrying out the objects of our invention in one form thereof, we employ a movable cover with a standing Wave detector haw'ng a movable pickup device of the slotted-section type. This cover is arranged to enclose the slot completely and is movable, thus permitting the probe to be moved to any desired position within the slot.

Our invention, together with further objects and advantages thereof, will be better understood from the consideration of the following description taken in connection with the accompanying drawings in-which:

FIGURE 1 shows an isometric view of one preferred type of slotted-section assembly, constructed according to our invention, in which an endless belt is employed as a cover for the slot.

FIGURE 2 shows a sectional view through the slotted section of FIGURE 1 on line 2-2.

FIGURE 3 shows a top view of a slotted-section assembly and movable cover according to a modification of our invention.

FIGURE4 shows a sectional View through the modifi- Zaiign of our invention shown in FIGURE 3 on the line Referring now more particularly to FIGURES 1 and 2, there is shown a slotted-section assembly comprising a wave guide 1 having a longitudinal slot 2 on its upper peripheral surface adapted to receive a movable probe 3 and insulator 4-. Probe 3 is slidable lengthwlse in slot 2 and may be advanced or retracted vertically in the slot by manipulation of a-means for probe control 5.

Probe 3, insulator 4, and control means 5 are carried by carriage member 6, which is disposed to be moved longi tudinally along slot 2 by means such as a rack and pinion assembly 7, operable by manipulation of knob 8.

A movable cover 9, having an aperture generally illustrated as of such size as to allow the insertion of probe 3 and insulator 4 through it, is disposed to keep slot 2 tightly covered at all times.

In the preferred embodiment shown in FIGURE 1, movable cover 9 is illustrated as an endless belt. This cover is made of conducting material, but we prefer that it be coated with a thin layer of nonconducting material in order to minimize noise. The cover is disposed in a recessed slot 11 of height andwidth sufiicient to accommodate the endless belt. Cover plate 12, which is detachable for ease in inserting and removing the slot cover, is disposed longitudinally through recessed slot 11 and covers substantially all of the endless belt in contact with the upper surface of wave guide 1. The lips 13 of detachable portion 12 are held in intimate contact with the recessed surface of wave guide 1 by screws "14 and define the guideway for the edge of cover 9. Movable cover 9 is also guided by rollers 15. These rollers ensure ease of movement of cover 9 and allow free travel of probe 3.

Movement of carriage member 6 carrying probe insulator means 4 exerts pressure on cover 9 and thus moves the cover as the probe assumes any desired longitudinal position in the slot. Thus, it can be seen that slot 2 is kept covered by cover 9 at all times, regardless of the position of probe in the slot.

Many modifications of this device may be made. For instance, the movable cover 9, instead of being in the form of an endless belt as shown, may be in the form of a strip of covering material which is rolled on springloaded rollers at each end of the slot in a manner somewhat analogous to the rolling means of a window shade. Such a modification could employ the driving means shown and would have the advantage of a more compact cover assembly.

Referring now more particularly to FIGURES 3 and 4, there is shown a modification of the present invention comprising a wave guide section 1 having a longitudinal slot on its upper peripheral surface adapted to receive "movable probe 3 and insulator 4. Probe 3- is slidable lengthwise in slot 2 and also may be advanced or retracted vertically by manipulation of a means for probe control 5. Probe 3, insulator 4, and control means 5 are carried by carriage member (not shown) which is disposed to be moved longitudinally along slot 2 by means such as a rack and pinion assembly (also not shown).

A flat, disc-like movable cover 16, having an. aperture (not shown) of sutficient size to allow insertion of probe 3 and insulator 4, is disposed to keep slot 2 tightly covered at all times. As shown, the cover 16 is mounted eccentrically with respect to the axis of probe 3 and is of sufiicient diameter to ensure that slot 2 is completely covered regardless of the longitudinal position of probe 3 in slot 2. Cover plate 17, juxtaposed the top surface 18 of cover 16, ensures intimate contact with the upper peripheral surface 20 of Wave guide 1. Cover plate 17 may be modified as a convex-shaped, disc-like member so as to effectively spring bias slot cover'16 against the top surface 20 of wave guide 1.

Longitudinal movement of probe 3, insulator 4, and control means 5 in slot2 causes slot cover 16 to be rotated eccentrically and to completely cover the slot at all longitudinal positions of probe 3. The phantom view in FIGURE 3 shows only the the cover plate 16a when probe 3 is atthe opposite end 'of the slot.

While we have shown and described particular embodiments of our invention, modifications and variations thereof will occur to those skilled in the art. We wish it to be understood, therefore, that the appended claims are intended to cover such modifications and variations which are within the true scope and spirit of our invention.

What we claim as new and desire to secure by,Letters Patent of the United States is:

1. In an ultrahigh-frequency apparatus for detecting standing waves in an electromagnetic energy transmission system, including a section of enclosed wave guide having a first aperture in the outer wall thereof, a detector device electrically coupled to and supported for longitudinal movement with reference to saidguide section for deriving a voltage indicative of the amplitude of the standing wave at any selected point along the length of said guide section, a movable conducting cover means disposed to shield said guide wall first aperture and having a second aperture of size sulficient for insertion of an insulated probe of said detector device into said guide, said means being of sufficient length on each side of said second aperture to cover said first aperture completely, said means being movable together with said detector device, the improvement comprising an endless belt as said movable conducting cover means, said belt disposed in a recessed slot of height and width sufficient to accommodate said belt, said belt disposed longitudinally through said slot to cover and be in contact with substantially all of the upper surface of said wave guide section, the position of said belt, including thev portion thereof juxtaposed the upper peripheral surface of said guide, being defined by a series of spring-biased rollers and said series of rollers so disposed that the longitudinal displacement of said belt is greater than the longitudinal displacement of said first aperture and less than the longitudinal displacement of said wave guide section.

2. In an ultrahigh-frequency apparatus for detecting standing waves in anelectromagnetic energy transmission system, including a section of enclosed Wave guide having a first aperture in the outer wall thereof, a detector device electrically coupled to and supported for longitudinal movement with reference to said guide section for deriving a voltage indicative of the amplitude of the standing wave at any selected point along the length of said guide section, a movable conducting cover means disposed to shield said guide wall first aperture and having a second aperture of size sufiicient for insertion of an insulated probe of said detector device into said. guide,

. said means being eccentrically rotatable about the. vertical axis of said insulated probe and being ofv sufficient diameter to cover said first aperture completely at all longitudinal positions of said detector device in said first aperture, said means being movable together lwithsaid detector device, the improvement comprising a relatively thin, convex-shaped, disc-like member as ,said movable conductor cover means, juxtaposed the top peripheral surface of said guide section and spring-biased against the upper peripheral surface of said guide section by apressure-exerting means disposed against the upper surface of said disc-like means, whereby the convex shape of said disc-like member provides the necessary spring-biasing forces.

References Cited in the file of this patent UNITED STATES PATENTS 2,571,055 Nordsieck Oct. 9,1951 2,772,402 Tomiyasu Nov. 27, 1956 2,811,201 Reid Oct. 29, 1957 FOREIGN PATENTS 567,287 Great Britain Feb. 7, 1945 831,569 Germany Feb. 14, 1952 897,739 Germany Nov. 23, 1953 940,051 Germany Mar. 8, 1956 

